Method of forming DRAM cell arrangement

ABSTRACT

A storage cell has a number of projections of a semiconductor substrate arranged in rows and columns, neighboring rows of the projections being translation-symmetrical in relation to a y-axis which extends parallel to the columns. Each of the projections has at least one first source/drain region of a selection transistor and one channel region arranged below the first source/drain region, which is surrounded by a gate electrode annularly. A storage capacitor is connected between the first source/drain region and a bit line. The bit line as well as the storage capacitor are arranged essentially above the semiconductor substrate. Second source/drain regions of selection transistors are buried in the semiconductor substrate and connected with each other. Word lines can be formed self-justified in the form of adjacent gate electrodes. The projections can be created by etching with only one mask. The storage cell can be produced with an area of 4F 2 , F being the minimal structural size that can be produced in the respective technology.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of application Ser.No. 09/274,733, filed Mar. 23, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a DRAM cell arrangement and a method for itsproduction. More particularly, the invention relates to a verticallystructured DRAM cell arrangement.

2. Description of the Related Art

In DRAM cell arrangements, i.e. memory cell arrangements with dynamicrandom access, what are known as single transistor memory cells are usedalmost exclusively. A single transistor memory cell comprises aselection transistor and a storage capacitor. The information is storedin the storage capacitor in the form of an electrical chargerepresenting a logical quantity of 0 or 1. By activating the selectiontransistor via a word line, the information can be read out via a bitline.

Usually, a first source/drain region of the selection transistor isconnected to the storage capacitor, and a second source/drain region ofthe selection transistor is connected to the bit line. A gate electrodeof the selection transistor is connected to the word line (cf. e.g.S.M.Sze Semiconductor Devices, AT&T Bell Laboratories, Murray Hill, N.J.1985, p. 487, FIG. 18a).

Since the memory density increases from generation to generation, thesurface of the single transistor memory cell must be reduced fromgeneration to generation. As such, reducing the dimensions of the memorycell is limited by the minimal structural size, that can be produced inthe respective technology. In addition, the memory cell may be altered.Thus, up to the 1 MBit generation, both the selection transistor and thestorage capacitor were realized as planar components. Since the 4 MBitmemory generation, a further surface reduction has had to occur by meansof a three-dimensional arrangement of the selection transistor andstorage capacitor.

One option is to design the storage capacitor in a trench instead of ina planar fashion (see K. Yamada, “A deep trenched Capacitor technologyfor 4 Mbit DRAMs”, Proc. Intern. Electronic Devices and Materials IEDM85, p. 702). However, the creation of such a buried storage capacitor isexpensive. Also, capacitor dielectrics with high dielectricity constantscannot be used, since their deposition is only possible in essentiallyplanar surfaces.

German Patent Document No. 195 19 160 C1 proposes a DRAM cellarrangement in which the storage capacitor is created over the selectiontransistor, and the bit line is buried in the substrate. Since thestorage capacitor is created at a surface of the substrate, capacitordielectrics with high dielectric constants can be used. Each memory cellhas a protuberant semiconductor structure having a first source/drainregion. A channel region is positioned underneath the first source/drainregion and a second source-drain region is positioned underneath thechannel region. The protuberant semiconductor structure is surroundedannularly by a gate electrode.

Semiconductor structures of memory cells are arranged in rows andcolumns. Neighboring rows of the semiconductor structures aretranslation-symmetrical in relation to an axis extending parallel to thecolumns. In order to create word lines in a self-justified fashion, i.e.without using masks that need to be justified, the intervals betweensemiconductor structures that are arranged along the columns are smallerthan intervals between semiconductor structures that are arranged alongthe rows. The semiconductor structures are surrounded by a grid-shapeddepression. The word lines emerge by the deposition and etchback ofconductive material in the form of gate electrodes that are situatedadjacently along the columns. The buried bit line is created from adoped layer, which is structured in a strip-shaped fashion by trenchesfilled with insulating material. The trenches are created with the aidof a strip-shaped first mask.

With the aid of a strip-shaped second mask, whose strips extendperpendicular to the trenches, depressions are created between thetrenches by the etching of semiconductor material. The depressions donot cut through the doped layer, so as not to cut through the bit line.The semiconductor structures emerge from a layer sequence by thecreation of the trenches and the depressions. The depressions are alsofilled with insulating material. The insulating material is subsequentlyetched back, the semiconductors are thereby exposed and the grid-shapeddepression emerges. Due to the common etching of the insulating materialin the depressions and of the insulating material in the trenches, thefloor of the grid-shaped depression is flat, which is essential for theself-justified creation of the word lines.

To increase the packing density, the first mask is created in thatstrips having a width F are first created by a photolithographicprocess. The strips are widened by the deposition and etchback ofmaterial. A distance between neighboring strips of the first mask isthus less than F. The strips of the second mask are created with thewidth F, by reason of which the above cited conditions regarding thedistances between the semiconductor structures are satisfied. The memorycell's surface area is 4F². The first source/drain region acts as thefirst capacitor electrode of the storage capacitor. A second capacitorelectrode is created by the whole-surface deposition of conductivematerial over the capacitor dielectric.

In U.S. Pat. No. 4,630,088 it is suggested that the storage capacitor beconnected between a first source/drain region of the selectiontransistor and the bit line. Each memory cell has a protuberantsemiconductor structure which is surrounded by a gate electrodeannularly. The memory cells are arranged off-set to one anotherdiagonally with respect to a word line direction. The storage capacitorincludes the first source/drain region, a part of awhole-surface-deposited capacitor dielectric, and a part of the bitline. The first source/drain region, a channel region and a secondsource/drain region of the selection transistor are arranged above oneanother in layers.

SUMMARY OF THE INVENTION

The invention is based on the problem of proposing a DRAM cellarrangement which, given a high packing density, can be produced with asmaller processing outlay than previously available. A production methodfor such a DRAM cell arrangement is also proposed.

In an inventive DRAM cell arrangement, a semiconductor substrateincludes a number of projections arranged in rows and columns.Neighboring rows of the projections are translation-symmetrical inrelation to a y-axis which extends parallel to the columns. Eachprojection has at least one first source/drain region and one channelregion of a vertical selection transistor, the channel region beingarranged under the first source/drain region. The projection can alsohave a second source/drain region of the selection transistor. Theprojection is provided with a gate dielectric at least in the area ofthe channel region. Each of the projections is surrounded annularly by agate electrode of the selection transistor. A word line is formed bygate electrodes which neighbor one another along an x-axis parallel tothe rows and which are situated adjacently. A first capacitor electrodeof the storage capacitor is electrically connected to the firstsource/drain region. The first capacitor electrode is cut in two by acapacitor dielectric of a second capacitor electrode of the storagecapacitor, which is arranged over the first capacitor electrode. Thesecond capacitor electrode is electrically connected to a bit line whichextends essentially parallel to the columns. The storage capacitor isconnected between the first source/drain region and the bit line. Thesecond source/drain region is buried in the semiconductor substrate.

In order to connect the second source/drain regions of selectiontransistors to a common potential, it is advantageous to connect atleast a few of them to each other.

Since the second source/drain regions are not connected to bit lines,they can be parts of a continuous layer which does not need to bestructured. A mask is not necessary for the creation of the secondsource/drain regions, which reduces the processing outlay.

Since not only the bit line, but also the storage capacitor is createdat a surface of the semiconductor substrate, the creation of structuresthat are buried in the semiconductor substrate is avoided, which alsoreduces the processing outlay.

The utilization of capacitor dielectrics with high dielectric constantsis possible, since the storage capacitor is not created in a deeptrench.

An advantage of the inventive memory cell arrangement is thatα-particles which emerge in the semiconductor substrate reach the bitline, which is arranged at the surface of the semiconductor substrate,and the storage capacitor having only a low probability, so that theprobability of falsifying information is therefore low.

The projections are preferably created by a structuring of a layersequence. To this end, a first layer which is doped by a firstconductivity type is created; a second layer which is doped by a second,opposite conductivity type is created over this layer; and a third layerwhich is doped by the first conductivity type is created over the secondlayer. The first layer, the second layer, and the third layer can becreated by implantation of the semiconductor substrate and/or by epitaxywhich is doped in situ. If a layer is created by epitaxy doped in situ,then the semiconductor substrate is enlarged by this layer.

To simplify the processing, it is advantageous if the projections arecreated by etching with only one mask. A grid-shaped depression is thuscreated which is divided at least by the third layer and the secondlayer. Thus, all the vertical dimensions of each projection—i.e.perpendicular to the semiconductor substrate—are equal. In other words,all the sides of each projection have the same length perpendicular tothe semiconductor substrate. Alternatively, parallel first trenches canbe created in a first (etching) step, and second trenches, which extendtransversely to the first trenches, can be created in a second(processing) step.

First source/drain regions of the selection transistors emerge from thethird layer as parts of the projections, and channel regions of theselection transistors emerge from the second layer. The grid-shapeddepression or the first trenches and the second trenches reach into thefirst layer. The second source/drain regions are parts of the firstlayer.

To simplify the processing, it is advantageous to produce thegrid-shaped depression or the first trenches and the second trenchesoptimally flat in that the first layer is not divided.

To simplify the processing, it is advantageous if the word lines arecreated self-justified. Self-justified creation is possible, forexample, if intervals between projections which are arranged along thecolumns and are parallel to the column direction, are greater thanintervals between projections which are arranged along the rows and areparallel to the row direction. To create the word lines, conductivematerial is deposited in a conformal fashion, whereby a wave-shapedconductive layer emerges. Valleys of the waves of the conductive layerextend parallel to the rows. The conductive material fills the spacebetween the projections which are arranged along the rows, while it doesnot fill the space between the projections which are arranged along thecolumns. The conductive layer is etched back anisotropically until partsof the conductive layer which are arranged above the first source/drainregions and between the neighboring projections along the columns areremoved. Thus, the word lines emerge from the conductive layer. The wordlines are not connected in the column direction. Since the conductivematerial fills the space between the projections which are arrangedalong the rows, there is relevant conductive material between theprojections which are arranged along the rows even subsequent to theetchback of the conductive material. At the sides of the projectionswhich are parallel to the direction of the rows, the word lines or thegate electrodes take the form of spacers.

To increase the packing density, it is advantageous if the distancesbetween the projections which are arranged along the rows are smallerthan F. To create the projections, a mask can be used which is enlargedby spacers in the row direction. For example, first parts of the maskare created in that material is deposited and structured by astrip-shaped first mask made of photosensitive resist, the strips ofwhich extend parallel to the columns. Additional material issubsequently deposited and etched back, whereby spacer-shaped secondparts of the mask emerge at the sides of the first parts of the mask.The first parts and the second parts of the mask are structured with theaid of a strip-shaped second mask made of photosensitive resist, thestrips of which extend parallel to the rows, whereby the mask emerges.The widths of the strips of the first and second photosensitive resistmasks as well as the distances between the strips of the firstphotosensitive resist mask and the distances between the strips of thesecond photosensitive resist mask are preferably equal to F. Thedescribed steps guarantee that the intervals between the projectionswhich are arranged along the rows are smaller than the intervals betweenthe projections which are arranged along the columns.

To simplify the processing it is advantageous if the first source/drainregion coincides with the first capacitor electrode. Alternatively, thefirst capacitor electrode is created over the first source/drain regionby the structuring of an additional conductive layer, for example.

To simplify the processing, it is advantageous if the second capacitorelectrode is part of the bit line. To this end, conductive material canbe deposited over the capacitor dielectric and structured in astrip-shaped fashion.

To enlarge the signal of the storage capacitor, it is advantageous ifthe capacitor dielectric is created from a material with a highdielectric constant, such as perovskite (e.g. Ba_(x)Sr_(1 −x)TiO₃,SrBi₂Ta₂O₃, PbZrTiO₃) or Ta₂O₅. Since such materials are preferablydeposited on flat surfaces, it is advantageous to create an insulatingstructure subsequent to the creation of the gate electrodes in thatinsulating material is deposited and planarized until the firstsource/drain regions are exposed. The capacitor dielectric is thendeposited over the first source/drain regions, which act as first.capacitor electrodes.

The semiconductor substrate is a silicon substrate or an SOI substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a substrate, subsequent to thecreation of a first layer, a second layer, a third layer, first parts ofa mask and second parts of the mask.

FIG. 1B is a cross-sectional view of the substrate which isperpendicular to the cross-section in FIG. 1A, subsequent to theprocessing steps in FIG. 1A.

FIG. 2A is a cross-sectional view of the substrate in FIG. 1A,subsequent to the creation of projections, first source/drain regions,channel regions, second source/drain regions, a gate dielectric, gateelectrodes and word lines.

FIG. 2B is a cross-sectional view of the substrate in FIG. 1B subsequentto the processing steps from FIG. 2A.

FIG. 3A is the cross-sectional view of the substrate in FIG. 2A,subsequent to the creation of an insulating structure, a capacitordielectric and bit lines.

FIG. 3B is a cross-sectional view of the substrate in FIG. 2B,subsequent to the processing steps from FIG. 3A.

FIG. 4 is a plan view of the substrate in which the positions of a firstand second mask made of photosensitive resist are illustrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The starting material is a substrate 1 of silicon, which is p-doped andhas a dopant concentration of approximately 10¹⁷ cm⁻³. An n-doped firstlayer S1 which is approximately 500 nm thick is created by epitaxy dopedin situ. The dopant concentration of the first layer S1 is about 10²⁰cm⁻³. A p-doped second layer S2 which is approximately 200 nm thick iscreated by epitaxy doped in situ. The dopant concentration of the secondlayer S2 is about 3×10¹⁷ cm⁻³. An n-doped third layer S3 which isapproximately 200 nm thick is created by epitaxy doped in situ. Thedopant concentration of the third layer S3 is about 10²¹ cm⁻³ (FIGS. 1Aand 1B). The substrate 1 is enlarged by the first layer S1, the secondlayer S2 and the third layer S3. A y-axis y and an x-axis x, which isperpendicular to the y-axis y, extend along a surface O of the substrate1.

To create a mask M1, silicon dioxide SiO₂ is deposited in a thickness ofabout 100 nm in a TEOS (“tetra-ethoxy-silane”) process. SiO₂ isstructured into strips with the aid of a strip-shaped first mask P1 madeof photosensitive resist (FIG. 4)—whose strips extend parallel to they-axis y, comprise a width of about 250 nm and have a distance of about250 nm from one another—whereby first parts M1 a of the mask M1 emerge.Subsequent to removal of the first mask made of photosensitive resistP1, SiO₂ is deposited in a thickness of about 80 nm and etched back,whereby second parts M1 b of the mask M1 emerge in the form of spacersalong sides of the first parts M1 a of the mask M1 (FIGS. 1A and 1B). Asuitable etching agent is CHF₃+O₂, for example.

SiO₂ is etched with the aid of a strip-shaped second photosensitiveresist mask P2 (FIG. 4)—whose strips extend parallel to the x-axis x,have a width of about 250 nm, and have a distance from each other ofabout 250 nm—whereby the first parts M1 a and the second parts M1 b ofthe mask M1 are further structured. The mask M1 thus emerges (FIGS. 1Aand 1B).

Subsequent to the removal of the second photosensitive resist mask P2,silicon is etched 600 nm deep with HBr+NF₃+He+O₂, dividing the thirdlayer S3 and the second layer S2. Cuboidal projections V emerge from thesubstrate 1, the distances of said projections from one another beingsmaller in the direction of the x-axis x than in the direction of they-axis y. The projections V are surrounded by a grid-like depression. Asparts of the projections, first source/drain regions S/D1 of verticalselection transistors emerge from the third layer S3, and channelregions Ka emerge from the second layer S2. Parts of the first layer S1which are arranged beneath the channel regions Ka act as secondsource/drain region S/D2 (FIGS. 2A and 2B). The mask M1 is removed byetching with CHF₃ and 0 ₂, for example.

To create a gate dielectric Gd, SiO₂ is grown in a thickness of about 5nm by thermal oxidation.

To create a first conductive layer, polysilicon which has been n-dopedin situ is deposited in a thickness of about 80 nm. The first conductivelayer is waved, the waves, i.e. their peaks and valleys, extend parallelto the x-axis x. The first conductive layer fills the space between theprojections V which are arranged along the x-axis x, while they do notfill the space between the projections V which are arranged along they-axis y. Polysilicon is etched back about 150 nm deep by etchback withC₂F₆+O₂, for example, exposing parts of the gate dielectric Gd on thefirst source/drain regions S/D1 and parts of the gate dielectric Gdbetween the projections V which are arranged along the y-axis y. Thefirst conductive layer is thus structured in the form of spacers atsides of the projections V which are parallel to the x-axis x. Gateelectrodes Ga emerge from the first conductive layer. The gateelectrodes surround the projections V annularly. Neighboring gateelectrodes Ga along the x-axis x are adjacent and form word lines. Theword lines extend parallel to the x-axis x (FIGS. 2A and 2B).

To create an insulating structure l, SiO₂ is deposited in a thickness ofabout 300 nm in a TEOS process and planarized by chemical-mechanicalpolishing, until the first source/drain regions S/D1 are exposed (seeFIGS. 3A and 3B).

A capacitor dielectric Kd is subsequently created in that bariumstrontium titanate is deposited whole-surface in a thickness of about 20nm (FIGS. 3A and 3B).

A second conductive layer is created by the deposition of AlSiCu in athickness of about 200 nm. AlSiCu is etched with BCl₃+Cl₂+CH₄, forexample, with the aid of a strip-shaped photosensitive resist mask whosestrips extend parallel to the y-axis y, bit lines B being created fromthe second conductive layer. The bit fines B have a width of about 250nm, and the distances between neighboring bit lines B are about 250 nm(FIG. 3A and 3B). The first source/drain regions S/D1 act as firstcapacitor electrodes of storage capacitors. Parts of the bit lines Bwhich are arranged above the first source/drain regions S/D1 act assecond capacitor electrodes of the storage capacitors.

The projections V are arranged in rows and columns. The x-axis x pointsin the row direction, and the y-axis y points in the column direction.Neighboring rows of the projections V are translation-symmetrical withrespect to the y-axis y. Neighboring columns of the projections V aretranslation-symmetrical with respect to the x-axis x.

There are many conceivable variations of the exemplifying embodimentwithin the framework of the invention. In particular, the dimensions ofthe described layers, projections, masks, spacers and bit lines can bearbitrarily adapted to the respective requirements. The same applies tothe proposed dopant concentrations.

Other materials can also be used for the capacitor dielectric. Forexample, an ONO layer 10 mm thick can be created in that an SiO₂ layeris created by thermal oxidation and silicon nitride is deposited overthis, which is partly oxidized by thermal oxidation. A differentconductive material can be used for the bit line, such as polysilicondoped in situ.

Also, the conductivity types of the doped layers can be reversed. Forexample, the substrate can be n-doped.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications may be madewithout departing from the spirit and scope of the present invention andwithout diminishing its attendent advantages. It is, therefore, intendedthat such changes and modifications be covered by the appended claims.

What is claimed is:
 1. A method for producing a DRAM cell arrangement,the method comprising the steps of: providing a semiconductor substratehaving an x-axis and a y axis extending along a surface of the substrateso that the y-axis is perpendicular to the x-axis; creating a pluralityof projections in said semiconductor substrate by etching with one mask,arranging said plurality of projections in rows and columns in whichneighboring rows of said plurality of projections aretranslation-symmetrical in relation to the y-axis extending parallel tothe columns and forming intervals between the plurality of projectionsalong the rows and the columns so that the dimensions of the intervalsalong the rows parallel to the x-axis are smaller than the correspondingdimensions of the intervals along the columns parallel to the y-axis;forming the mask by depositing a material and structuring the materialinto strips by a strip-shaped first photosensitive resist mask andextending the strips parallel to the y-axis thereby creating first partsof the mask, depositing additional material and etching back thematerial until second parts of the mask emerge as spacers at the sidesof the first parts of the mask, structuring the first parts and thesecond parts of the mask by a strip-shaped second photosensitive resistmask and extending the strips parallel to the x-axis, the distancebetween the strips of the second photosensitive resist mask beingidentical to the distances of the strips between the firstphotosensitive resist mask so that the mask emerges and creates theplurality of projections; arranging at least one first source/drainregion and a channel region of a selection transistor in each of saidplurality of projections, said channel region being positioned undersaid at least one first source/drain region; providing a gate dielectricat least in the region of said channel region; surrounding each of theplurality of projections annularly with a gate electrode of theselection transistor by depositing a conductive material in anessentially conformal fashion in a thickness that spaces between theprojections arranged along the rows are filled with the conductivematerial, but spaces between the projections arranged along the columnsare only partially filled, and anisotropically etching back theconductive material until parts of the conductive material situated overthe first source/drain region and between neighboring projections alongthe columns are removed; creating a word line in the form of adjacentgate electrodes which neighbor one another along the x-axis parallel tothe rows; burying a second source/drain region of the selectiontransistor in the semiconductor substrate under each channel region;electrically connecting a first capacitor electrode of a storagecapacitor to the first source/drain region; providing a capacitordielectric on the first capacitor electrode; and electrically connectinga second capacitor electrode of the storage capacitor to a bit line andextending said bit line essentially parallel to the rows on thecapacitor dielectric.
 2. The method of claim 1, wherein the step ofcreating said plurality of projections further comprises the steps of:arranging a first layer doped with a first conductivity type on thesemiconductor substrate; arranging a second layer doped by a second,opposite conductivity type, on said first layer; arranging a third layerdoped by the first conductivity type on said second layer; and creatingeach of said plurality of projections so that the third layer and thesecond layer are divided.
 3. The method of claim 1, further comprisingthe steps of: depositing and planarizing an insulating materialsubsequent to creating said gate electrode until said at least one firstsource/drain region is exposed; and creating a conductive layer in astrip-shaped fashion so that bit lines emerge extending parallel to they-axis and covering the at least one first source/drain region, saidconductive layer acting as a second capacitor electrode above the atleast one first source/drain region.